Capacitors are normally charged and discharged through a resistor. The time required for the capacitor to charge or discharge is dependent upon the value of the related resistor. Therefore, if a capacitor is charged to a known level and subsequently allowed to discharge through a resistor, the resistance can be calculated by measuring the time required for the capacitor to discharge.
Modern microprocessors provide outputs that are capable of switching their voltage level from a high level to a low level. The high level is typically 5 volts dc and the low level is typically 0 volts dc. Microprocessor output can then be connected to circuitry that causes a capacitor to charge or discharge, depending on the state of the microprocessor output.
Subsequently, the microprocessor can determine a value relative to a variable resistive sensor. An example of this is shown in U.S. Pat. No. 4,841,758 to Levine et al.